1. Field of the Invention
The present invention relates to an electrophotographic process and an electrophotographic apparatus and, more particularly, to an electrophotographic process using an amorphous silicon base photosensitive member (a-Si photosensitive member) and an electrophotographic apparatus having the photosensitive member.
2. Related Background Art
The a-Si photosensitive members have characteristics of high surface hardness, high sensitivity to long-wavelength light of semiconductor lasers (770 nm to 800 nm) etc., little deterioration after repetitive use, and so on and are commercially used as photosensitive members for electrophotography, particularly, for high-speed copying machines, LBPs (laser beam printers), and so on.
FIG. 1 is a schematic, structural view for explaining an example of the image forming process in an electrophotographic apparatus using the a-Si photosensitive member, in which around a photosensitive member 401 arranged to rotate in the direction of arrow R1 there are provided a primary charger 402, an electrostatic latent image forming section 403, a developing unit 405, a transfer sheet supplying system 406, a transfer charger 407a, a separation charger 407b, a cleaner 409, a conveying system 410, a charge-eliminating light source 411, and so on. Normally, a corona charger excellent in uniform charging is widely used as the primary charger 402.
The image forming process will be described below with the above example. The photosensitive member 401 is uniformly charged by the primary charger 402 to which the high voltage of +6 to 8 kV is applied. Light is guided from the electrostatic latent image forming section 403 to be projected onto the photosensitive member 401 to form an electrostatic latent image thereon. A negative toner is supplied from the developing unit 405 onto the latent image to form a toner image. On the other hand, a transfer sheet P is supplied through the transfer sheet supplying system 406 to the photosensitive member and a positive electric field, which is of a polarity opposite to the polarity of the toner, is applied thereto from the back in a gap between the photosensitive member 401 and the transfer charger 407a to which the high voltage of +7 to 8 kV is applied. This causes the negative toner image on the surface of the photosensitive member to be transferred onto the transfer sheet P. The transfer sheet P is separated by the separation charger 407b to which the high AC voltage of 12 to 14 kvp-p and 300 to 600 Hz is applied and it is conveyed through the transfer sheet conveying system 410 to a fixing device (not illustrated), in which the toner image is fixed. The transfer sheet P is then discharged out of the apparatus.
In the electrophotography, a photoconductive material for forming the photosensitive layer in the photosensitive member needs to have the following characteristics; high sensitivity, high SN ratio [photocurrent (Ip)/dark current (Id)], an absorption spectrum compatible with spectral characteristics of electromagnetic waves to be radiated thereto, quick optical response, and desired dark resistance, and harmlessness to the human body during use, and so on. Particularly, in the case of the photosensitive members for image-forming apparatus incorporated in the image-forming apparatus used as business machines in offices, a nonpolluting property during the aforementioned use is a significant point. One of the photoconductive materials demonstrating excellent properties in the aforementioned aspects is hydrogenated amorphous silicon (which will be referred to as "a-Si:H"). For example, Japanese Patent Publication No. 60-35059 describes an application thereof to the photosensitive member for image-forming apparatus.
The photosensitive members for image-forming apparatus using a-Si:H are generally made by heating a conductive support at 50.degree. C. to 400.degree. C. and forming a photoconductive layer comprised of a-Si on the support by a film forming method such as a vacuum evaporation method, a sputtering method, an ion plating method, a thermal CVD method, a photo CVD method, a plasma CVD method, or the like. Among them the plasma CVD method, which is a method for decomposing a source gas by direct current or high-frequency or microwave glow discharge to form an a-Si deposited film on the support, is practically used as a preferred method.
For example, Japanese Patent Application Laid-Open No. 54-83746 suggests the photosensitive member for image-forming apparatus comprised of an electroconductive support and a photoconductive layer made of a-Si containing halogen atoms as a constituent (hereinafter referred to as "a-Si:X"). This application describes that when a-Si contains 1 to 40 atomic % halogen atoms, the photoconductive layer has high heat resistance and good electrical and optical characteristics as a photoconductive layer of the photosensitive member for image-forming apparatus.
Japanese Patent Application Laid-Open No. 57-11556 describes the technology of forming a surface layer of a non-photoconductive amorphous material containing silicon atoms and carbon atoms, on the photoconductive layer of an amorphous material containing silicon atoms as a matrix in order to improve the electrical, optical, and photoconductive properties including the dark resistance, photosensitivity, optical response, and so on, operating environment characteristics such as humidity resistance and the like, and temporal stability of the photoconductive member having the photoconductive layer comprised of the a-Si deposited film.
Further, Japanese Patent Application Laid-Open No. 60-67951 describes the technology of the photosensitive member in which a light transmissive insulating overcoat layer comprised of amorphous silicon containing carbon, oxygen, and fluorine is stacked, and Japanese Patent Application Laid-Open No. 62-168161 describes the technology using an amorphous material containing silicon atoms, carbon atoms, and 41 to 70 atomic % hydrogen atoms, as a surface layer.
Further, Japanese Patent Application Laid-Open No. 57-158650 describes that the photosensitive member for image-forming apparatus with high sensitivity and high resistance can be obtained by using a photoconductive layer comprised of a-Si:H containing 10 to 40 atomic % hydrogen, wherein the absorption coefficient ratio of absorption peaks at 2100 cm.sup.-1 and at 2000 cm.sup.-1 in an infrared absorption spectrum is 0.2 to 1.7.
On the other hand, Japanese Patent Application Laid-Open No. 60-95551 discloses the technology for preventing lowering in surface resistance due to adsorption of water in the surface of the photosensitive member and image smearing occurring therewith by carrying out the image forming process including charging, exposure, development, and transfer while maintaining the temperature of the vicinity of the surface of the photosensitive member at 30.degree. C. to 40.degree. C. in order to improve the quality of image of the amorphous silicon photosensitive member.
These technologies improved the electrical, optical, and photoconductive characteristics and the operating environment characteristics of the photosensitive members for image-forming apparatus and also improved the image quality therewith.
Further, in order to prevent and eliminate the aforementioned image smearing of the photosensitive member at a high humidity, it is known to heat the photosensitive member by a heat source disposed on the internal surface side of the photosensitive member, for example, by an electric heater of a surface or rod shape disposed on the internal surface side of the cylindrical photosensitive member.
However, the continuous heating by the heater for prevention of the image smearing increases power consumption, as described above. Capacities of such heaters are normally approximately 15 W to 80 W, which do not always seem large electric energy. In most cases the apparatus is always powered throughout the day including the nighttime. The power consumption per day for the heating could reach 5-15% of the total power consumption of the image-forming apparatus in certain cases.
Incidentally, with progress in increase of functions of the electrophotographic apparatus and in space savings of offices etc., there have been increasing desires for machines having the space saving effect, multiple functions, and high copy speed. It is thus necessary to design the apparatus with consideration to the tendency toward high speed, downsizing, and multi-functioning from the designing aspect.
With progress in the tendency toward the high-speed operation, downsizing, and multi-functioning of the electrophotographic apparatus, however, the charging device becomes smaller and the process speed becomes higher. These factors decrease the pass time of the photosensitive member in the charger and this makes it difficult to achieve high charging on the surface of the photosensitive member, i.e., to charge the surface of the photosensitive member sufficiently. From the aspect of energy savings, there are also desires for further decrease of power consumption of the entire electrophotographic apparatus by cutting off the drum heater and by lowering the current value of the charger.
Particularly, where the speed is increased further or where the size of the photosensitive member is decreased further, there will arise a significant problem as to the charging. In the case of the speed increase, even if the width of the charger is kept equal, a time in which a certain point of the photosensitive member passes through the inside of the charger, that is, a time for charging thereof, becomes shorter, so that a charge amount is decreased in some cases. In cases where the diameter of the photosensitive member of a drum shape is decreased, the width of the charger is limited thereby, and as a result, a sufficient area is not assured for the charging. This would result in failing to achieve the sufficient charging in some cases.
Another problem common to the increase of the operating speed and the decrease of the diameter of the photosensitive member is a decrease of the time in which a certain point of the surface of the photosensitive member moves from the exposure site to the charger, for the next charging of the surface of the photosensitive member. When amorphous silicon is used, the photosensitive member has the optical memory phenomenon due to exposure. Since this optical memory decreases with a lapse of time after the exposure, it is more apt to appear as a ghost in an image as the aforementioned time becomes shorter. In order to remove this ghost, it is possible to effect excessive charge-eliminating exposure, but chargeability becomes more apt to be degraded with increase of the light quantity of the charge-eliminating exposure.
If there is large temperature dependence of the characteristics of the photosensitive member even after these problems have been overcome, the temperature control of the photosensitive member by the heater cannot be omitted.
Thus, in designing the image-forming apparatus utilizing the electrophotographic process and the electrophotographic image-forming process, it is necessary to achieve improvements from the total viewpoint in the electrophotographic, physical properties and mechanical durability of the photosensitive member for image-forming apparatus so as to solve the above problems and also achieve further improvements in the charging device capable of uniformly charging the photosensitive member with high charging efficiency and in the image-forming apparatus.
Further, diameters of dots are decreased for the purpose of improving the image quality. In this case, it is necessary to enhance reproducibility of dots and it is also important to improve it in the level of the latent image.